I use really big zip ties. There is a couple of slots that I snaked the zip tie through. I crammed some foam pieces around the battery to give it a tighter fit. I've flopped mine of its side 2 times and the battery has never moved. Its not the best way but it works.
I dont have pics but i used a small piece of garage door insulation between my battery and the pocket under the hood. I did it once to test fit and couldn't get the battery back out so I left it. 2 years and 4k miles with no trouble yet.